The present invention relates generally to an apparatus and process for producing an extruded plastic material product and, in particular, to an extrusion process for producing a thermoplastic material product having a foamed cellular core and an integral, non-porous skin.
Production of foamed thermoplastic material products by a continuous extrusion process has become increasingly popular. It has been found that the continuous extrusion process offers significant advantages of economy and versatility over the more common foam injection molding process. While commercial activities have concentrated on the lower cost thermoplastic materials such as polystyrene, polyethylene, and polyvinylchloride, there has been considerable development activity in polypropylene, acrylonitrile butadiene-styrene, polyamides and polycarbonates.
One of the most common methods of producing extruded foamed thermoplastic material is known as the freefoaming extrusion technique. This process is similar to conventional solid plastic extrusion except that the thermoplastic melt material contains a blowing agent which causes the melt material to foam and expand as the melt material emerges from an extruder die. In this process, the extruder die is constructed to produce a foamed thermoplastic material having a cross-sectional profile which is an approximation of the desired cross-sectional profile. The foamed thermoplastic material is then fed through a sizer which shapes the foamed thermoplastic material into a product having the desired cross-sectional profile. While this technique is typically capable of producing a foamed thermoplastic product having a uniform distribution of cells of the desired size, the resulting skin tends to be rather porous.
In order to obtain an extruded foamed thermoplastic material product with an outer solid or non-porous skin which surrounds an inner cellular core, a technique known as the controlled foam extrusion process is used. Examples of such a process can be found in U.S. Pat. No. 3,764,642 to Boutillier. The controlled foam extrusion process consists of extruding a foamable thermoplastic melt material uniformly containing a blowing agent through a specially designed extruder die and into a cooled shaper which is adjacent the die. The shaper is provided with a longitudinally extending cooled internal metal surface adapted to form the desired cross-sectional shape in the extruded melt material. In the shaper, the outer surface of the extruded melt material is rapidly cooled as it comes in contact with the metal surface to prevent foaming of the melt material adjacent the outer surface, thereby forming an outer integral solid skin. At the same time, the inner portion of the extruded melt material expands to form an inner cellular core.
In some instances, the extruder die can be designed to produce extruded foam products having a solid skin along one portion of the outer surface of the product and a cellular portion along the remaining portion of the outer surface. Such an arrangement is disclosed in U.S. Pat. No. 3,879,505 to Boutillier et al.
In the controlled foam extrusion process described above, the outer integral skin is formed about the inner cellular core by rapidly cooling a portion of the extruded melt material prior to the reaction of the blowing agent. Immediately after leaving the extruder die, an outer portion of the melt material contacts the cooled inner metal surface of the shaper and is pressed against this cooled surface by the expansion of the remaining inner portion of the melt. The cooled inner surface of the shaper is maintained at a temperature which enables the melt material in contact therewith to be cooled to a temperature below the blowing temperature of the melt material, thereby preventing expansion of this portion of the melt material to produce an outer, integral skin.
The thickness of the skin produced by this process corresponds to the depth in the melt material which is cooled to a temperature lower than the blowing temperature prior to the reaction of the blowing agent. Thus, in order to produce a relatively thick skin, it is necessary to maintain the temperature of the inner forming surface of the shaper substantially below the associated blowing temperature. It will also be appreciated that the inhibited blowing agent in this process remains in the skin portion ready to react if the skin is subsequently exposed to heat.